Method and apparatus for reducing echo effects in picture transmission systems



Oct. 2, 1945. F. J. BINGLEY ETAL 2,386,083 METHOD AND- AIPARATUS FORREDUCING ECHO EFFECTS l IN PICTURE TRANSMISSION SYSTEMS Original FiledMarch 6, 1942 4 Sheets-Sheet 1 DHRK'EcHo l'L/qm" EcHo furba-amar mzFHME. N2 Hem N2- WMW? I .Oct.2, 1945. lr-'..LILaIICYLEY ETAL S'2,386,088

METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS y f IN'PICTURETRANSMISSION SYSTEMS Original Filed March 6, 1942 4 Sheets-SheetZ@IDMIVLLy `e Q VIDEO,

SYNC/mN/zmg@ /27 miga@ @MHWJ @bmw/62m F. J. BINGLEY Er'Al.

Oct. 2, 1945.

` .METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURETRANSMISSION SYSTEMS 4 Sheets-Sheet 3 Original Filed March, 1942NOROMEMQ 4 Wwubl lll Il.

QNUSS boum lill QSMQQU ,OCL 2, l945. F. J. NGLEY, ETAL 2,386,088

METHOD AND APP TUS FOR REDUCING ECHO EFFECTS v IN PICTURE TRANSMISSISYST S Original Filed Marc 19 4 Sheets-Sheet' 4 U V1 HV1-'Wm'.- 4 1 1. A

v@ V W/ MM Athe drawings by the closely spaced dots.

,tain of the details of the system of Fig. 6; and

Fig. 8 is an explanatory diagram illustrating the Aoperation of thesystem of Fig. 7.

The present'invention may best be understood of the video portion of thesignal camby reason assaose of their lesser amplitude, be ignored, and,consequently, formost practical purposes only those `echoes produced bythe higher amplitude blankby considering first the causes of echosignals, I

and their appearance when viewed on the-screen of the picture tube. Atypical condition, which may give riseto objectionable echoes isillustrated in Fig. 1 In this figure, a television receiver R isrepresented as being 12'mi1es distant from a transmittingstation T. Atdistances of 6 and 10 miles from the transmitter and receiver,respectively, is a wave reflecting structure such as a tall building, a.water tower, a steel bridge,l or the like. Taking the speed of a radiowaveas about one-fth mile per microsecond, it will be apparent thattl'ietime required for the signal to traverse the direct 12-mile pathbetween the transmitter and receiver will be about 60` microseconds,while the time required for the rellected signal to traverse theindirect 16-mi1e path will be about 80 microseconds. Consequently, thereected wave, i. e., the echo, will arrive at the receiver about 20microseconds behind vthe direct wave. This may be regarded asillustrative of a'simple echo, as differentiated from multiple echoeswhich arise when a pluralityof reecting structures provide a pluralityof indirect signal paths of different length.

Fig. 2 is an explanatory diagram in which the time and amplitudecharacteristics of a typical television signal S are related to thescreen. P of a conventional television picture tube. 'I'he televisionsignal, in accordance with the present practice, may comprisesynchronizing pulses Ss of approximately 5.1 microseconds duration,blanking pulses Sb ofapproximately 10.2 microseconds, and a line orvideo period Sv of approximately 53.3 microseconds,. These aresubstantiallythe specications employed in a conventional 525-lineinterlaced television system, with 30 frames (60 elds) per'second. Sincethe line 4the screen P has, for convenience, been made equal in lengthtothe line 'portion of the televing and synchronizing pulses need beconsidered.

Of these` pulses, the latter are, of course, the more important byreason of their greater amplitude.

Line B in Fig. 2 is illustrative of the normal appearance of a pictureline when receiving a strong echo signal. 'The echo illustratedresembles those which are produced by the delayed reception of theblanking and synchronizing signal, and is displaced -from the left-handedge of the screen P by a distance which is proportional to the timeinterval between the arrival of the signal traveling the direct path andthe arrival of the signal traveling the indirect or reflected path. Ifthe difference in length between these paths is small, the echo will bereproduced at or near the left-hand edge of the screen, whereas greaterpath differences cause the echo to appear further tothe right. Ofcourse, where the path' difference is very great, the attenuationsuffered by the reflected signal is usually so considerable that theecho is too weak to be noticeable.

As is indicated in the drawings, the echo shown in line B is produced bythe arrival of blanking and synchronizing pulses during the line orvidec period Sv. Specifically, this echo is produced by the arrival ofthe pulses S, Sb, by way of an indirect path, approximately 20microseconds after the reception of these pulses by way of the directpath. AThe echo signal produces on the screen P an echo pattern Whosewidth is equal to the width of the line, and whose length is equal to lthe distance traveled by 'the scanning beam in 10.2 microseconds, theduration of the combined blanking and synchronizing signal. The20-second delay chosen for this illustration, it will be recalled, isapproximately the delay produced as a result of a reected signaltraveling an additional 4 miles as illustrated in Fig. 1.,

. Whether the echo, as it appears upon the picture tube screen, will bea dark echo or a light echo, depends upon the phase relation visionsignal S. As is conventional, the ampli-` tude of the blanking pulses Sbmay be regarded as corresponding to a black signal level, zero` carrieras corresponding to avery bright signal level, while the synchronizingpulses may be regarded as corresponding to a blacker-than-black orinfra-black signal level. The video signal existing during the 53.3second line'period has, for the purpose of this description, beenestablished at a level midway between zero carrier and the black level,and will correspond approximately, therefore, to a gray level.

vWhen a signal of these characteristics is received without an attendantecho, there will b e reproduced, upon thescreen'of the picture tube, aline similar to that shown (with exaggeratedy width) at A inY Fig. 2,and having a uniform the desired direct signal. `Strictly speaking, un-

der these conditions, the appearance of the entire line will be aiected,but in general echoes between the video carrier and the echo carrier atthe'point of detection. If these carriers arrive more or less in phase,the resultant R. F. signal supplied to the detector will be greater thanthe amplitude of the video carrier alone, and, consequently, thecombined signal will tend toward the black level and a dark echo will beproduced as shown in line B. Thev darkest part of-th'e echo will be thatcentral portion which vcorresponds to the synchronizing pulse Ss, while`the outer portions correspond to the blanking signal d will be somewhatless dark, but darker than that part of the line which is not distortedby echo; On the vother hand, if the video and echo carriers arrive ingenerally opposite phase, the reverse rwill be true, and a light echosuch as that shown in line C will result.

Attention is now directed to Figs. 3 and 4 which illustrate certain ofthe methods which may be employed, in accordance with the presentinventionfto elect substantial cancellation of echo patterns. No attempthas been made in these gures to maintain the identical scale employed-ln Fig. 2. Moreover, for simplicity, only the echo caused by thedelayed arrival of thesynchronizing signal vis illustrated, the lesserecho produced by the blanking signal being omitted. Of the numerousechocancellation schemes that we have times.

gaseosa developed, with the present invention as a basis,

Vperhaps one of the simplest is illustrated in Fig. 3.

The assumption in Fig. 3 is that transmission is carried out inaccordance with the single-carrier system, i. e., where video andsynchronizing signals are transmitted on acommon carrier frequency.Assume that a dark echo, such as is shown in frame No. 1 of Fig. 3, isobtained. This echo extends from top to bottom of the`picture as shown,for each individual line of the frame includes an echo of the type shownin detail in line B of Fig. 2, and the echo in each of the lines willbe, of course, in substantially identical positions relative to the edgeof the picture.

manner, frame No. 2, and all succeeding frames,

would present the same appearance, so far 4as echo is concerned, asframe No. 1. According' to the present invention, however, transmissionis carried out in such a manner that a periodicl reversal of` phaserelation takes place between the echo carrier" and the video carrier,causby a frame having a light echo, such as that shown in frame No. 2 ofFig. 3. This may be accom- .plished by the transmitting station bychanging If ,transmission were carried out in the conventional ing eachframe having 'a dark echo' to be followed` carrier phases or polaritiesat predetermined .Y

For example, all odd-numbered frames might be transmitted in theconventionalmanner with no carrier phase changes, while all even-`numbered frames mightbe so transmitted that the carrier phase obtainingduring the synchronizing (or blanking and synchronizing) lintervals isreversed with respect to the carrier phase obtaining during the videointervals.

This may bestbe understood in connection with Vthe composite signalrepresentation of Fig. 5 (a) which is intended to illustrate, in reducedscale, a television signal of the type shown in the lower portion ofFig. 2. The\signals to the left of the dashed line maybe considered asthose producing the scan shown as frame No.V 1 in Fig. 3, whilethesignals to the right of the dashed line vare those producing the scandenoted frame No.

2 in Fig. 3. In `each frame the echo is produced,

` as hereinbefore described, bythe delayed, ar-

rival of echoes of the synchronizing and blanking signals'. In Fig'.5(a) the absence of crosshatching in the signals to theleft of thedashed lines-s: is intended to indicate that no phase shift is producedin the carrier asbetween the of dark and light echoes upon the screen ofthe picture tube is substantially that which would obtain if no -echoeswere `being reproduced at all.4 In a conventional television systembased on 30 complete frames per second, there will be iifteen completeecho alternations" per second, each alternation consisting of one framehaving a dark echo followed by a frame with a light echo. Apparently therapid substitution of light echoesffor dark echoes, and vice versa.causes the e eye to average the echoeffects and to substan-v tiallyignore the individual echo" images themselves. i We have found thatcancellation of echoes can be made even more veiective if, in eachframe,

the echo is broken up into a series of alternate light andgdark areas asillustrated in .Fi-g. 4. This figure shows a pattern that mayadvantageously be employed in a conventional system. A pattern of thischaracter may be `obtained by using a signal inwhich carrier phasereversal is" effected at the transmitter at approximately the beginningand the end of each of the crosshatched intervals shownin thesignalrepresen-l tations (b) to(e) of Fig. 5. With asignal of this type',thecharacter of the echo (i. e., whether dark orlight) changes forsuccessive lines in time sequence. In a conventional interlaced scanningsystem, theeiectproduced will be similar to that illustrated in Fig.` 4in whichthe lines are numbered from l to I B intime sequence for twocomplete frames, i; e., four complete iields. relation between lthesynchronizing signal echo In line l, it is assumed that the phase andthe video carrier is such that a darkv echo results. In line 2 (which inan interlaced system ris spaced from line l by the width of one line),

' this phase relation is reversed to produce a light echo,reversed againin line 3 to produce a dark field. y'Ihe second field, comprisinglines-G to 9,

i. is transmitted without change in -sequence so that the scanningpattern, or raster as itzis called.V

consists of a plurality of pairs of lines with alterecho, and soon forlive lines to produce the first nately dark and light echoes, asillustrated in Fig. 4, frameNo. 1. In'frame No. 2, this process iscontinued'without change in sequence, the rst eld of frame 2,compris'inglineslll to Mr, while the second eldcomprises lines i5 to I8.

video and the synchronizing and blanking intervals, whereas in thesignals shown to the'right' of :1r-m, the cross-hatched blanking andsynchronizing signals are intended to indicate that the phase of thecarrier wave, during the blanking and synchronizing intervals,'isreversed with respect to the pha'seof the carrier during the videointervals.

frame No.` 1, as to' produce a dark echo, it follows that during frameNo. 2 (with relatively reversed phase relationships) light echoes willbe produced. `The characteristic .of ythe echo (whether dark or light)during the video intervals of frame No. 1 is indicated arbitrarily inConsequently; if the delay synchro-` nizing signal echoes arrive in suchphase, during Fig. 5 (a) by the circled plus signs; whereas, lduringframe No. 2, circled minus signs are employed to indicate that the echowould have a different characteristic as a result of the reversal inphaseA the observer, the ,Jop-

It will be seen, however, that because each frame includes an odd numberof lines, the dark echoesL in frame No. 2 occupy those` parts of theraster which in frame No. 1 'were occupied by the light echoes. andconversely. Since these frames are eii'ectively superimposed at shortintervals in transmission, the echoes tend to cancel as far as theobserver is concerned. 'Ihe system illus'- trated in Fig. 4 has theadvantage over that in Fig. 3 thatit breaks up the echo signal socompletely-as to substantially eliminate. all trace of ickering in eventhoselocations where echo sigvnals are very strong. Eiectively,the'system of Fig. 4 interlaces the ec'hoes in both time and .i

- space relation.

It has already been explained, in general, howV echo cancellation maybeobtained by successive- 1y reversing the phase ofthe echo with respectto the picture carrier in time relation. Specic examples showing justwhen. these phase reversals may be made, to secure echo cancellation ofthe type described with reference `to Fig. 4, are

illustrated in Fig. 5 (b) to (e) inclusive. In these illustrations,the'cross-hatched portions of the signals may beregarded as representingone-ar-l bitrary phaserelation, the open portions o f the signalsrepresenting a lsubstantially' opposite phase relation.

In Fig. (b), at time t1, the carrier phase is,

end of the manning fblanking and synchronizing interval so as 'to avoidanypossibilityoi impairment of interlacing. Preferably, the phasechanges are suspended for the nine lines following the beginning of thevertical blanking period.

It is not necessary that the desired phase changes be eiected preciselyat the beginning and end of the selected blanking pulses as shown in'Fig. 5 (b). If desired, these chan-ges may be made to occur atthe-beginning and end of the synchronizing pulses', or at some timewithin the blanking signal intervals preceding and succeeding thesynchronizing-signals themselves. The latter system oi timing isillustrated in Fig. 5 (e). Where phase changes are effected to coincidein time with the synchronizing signals, rather than with blankingsignals, it follows that echo cancellationv will be secured only forthesynchronizing signal echoes, but since echoes of the synchronizingsignals are by far the most important,

- the choice between the various timing sequences may befound to belargely one of convenience. Referring generally to the signalsrepresented in Fig. 5 at (b) and (e), Iit will be seen that every otherblanking and synchronizing signal istransmitted with its carrier phasereversedrelative Ato the phase of the carrier during the rest of'thetelevision signal. Consequently, ifV the synchronizing pulse transmittedin the interval ti -.t2 is received as an echo during the immediatelyfollowing video line interval, an echo image will appear on thetelevision screen for that particular line.` Ii' this echo be a lightone, indicated by the circled minus sign, the echo in thenfollowing line(intime sequence) will be a-dark one, as is indicated by thecircled'plus sign. That the sign" of the echowill be'dierent in thetwo-cases will be seen from the fact that in one case an echo of onephase will beat with a video line of opposite phase, whereas in theother case, the echo and video line are of like-phase.' Thus, the echoimage alternates from dark to light to dark, and

. the synchronizing pulses themselves.

5 (c). Here the phase oi the carrier is reversed after alternatesynchronizing or blanking pulses. i. e., afterevery second pulse. Thus,at t1, the end of the first blanking interval, the carrier phase isreverse, but no further reversal in relative phase occurs until time tz.which corresponds to thev end of the third blanking intervaLand again atta, the y end of the fifth blanking interval, and so on. Here againtheecho image will alternate from light to dark, etc.. as indicated by thecircled plus and minus signs.

In Fig. 5 (d) the carrier phase is reversed for --alternate video (line)periods, the phase reversals taking piace at times t1, tata. etc., asindicated. It will be seen that this procedure will also produce analternating echo similar to those produced by the signals of Fig. 5 (b)to (e) inclusive.

In all of thesevariations. it should be understood that the phasechanges arenot necessarily made precisely at the beginning and/or Aendof the blanking periods, but may be eilectedI within the blankingperiods. as illustrated .in Fig. 5 (e), or may coincide with thebeginning and/or end of Similarly, it shouldbe understood that theinvention is not limited to the speciiic methods of echo cancellationillustrated in Fig. 5, since other suitable sequences of phase reversalmay be utilized by those skilled in the art without departing from themethods and teachings of this invention.

From the illustrations of Fig. 5, it will be seen that the carrier phaseis preferably reversed at least `time-s per second, where L is thenumber of picture lilies per frame and F is the number of completeframes transmitted per second.

O ur invention may be put into effect by means of the transmittingsystem 'shown diagrammatically in'Fig. 6. Ant-oscillator I 9 serves asthe primary source of can'ien signal. If desired, this so on, from lineto line in time sequence, this being indicated in Fig. 5 (b)y thealternating plus and `minus signs.n m y In vthe roregolng, cases havebeen described where the echo arrives'alternately in-phase andout-of-phase with the videovsignal. Obviously, of course, there will beinstances wherein the echo will arrive alternately, leadingand'laggingthe video' signal, forexample, by 90 degrees.- Where thisoccurs, the echo is of little importance, since the resultant of astrong signal (the direct signal).

.and a weak signal (the echo) differing l90 in phase is notsubstantially different in magmtue from the strong-signal.

Another sphase changing sequence capable o! ,l me.

ent invention may be produced in the transmitproducing-an echo imagewhich alternatesfrom line to line in time` sequence, is illustrated inFig.

source may operate at a 45 sired carrier frequency,

f' sesqui-side-band illter 2l,

Amplitude modulation of submultiple of the dethe desired carriertrequency` being obtained by passing the wave from the source I9 througha suitable frequency multiplier circuit l2|). .v unit'20 may thenreversing means 2| whose reversing operation is controlled in responseto a keying signal from a source 22. The operation and construction ofthe units 2| and 22 will be described in detail hereinafter.k Thecarrier signal output o1- the unit 2| may then be passed through asuitable modulated amplier stage 23, thence through the antenna orradiating system ,25.

the `carrier wave may of a conventional modsupplied with video,synchronizand blanking signals from the source 21. `The 'phase reversalscontemplated in the presbe accomplished bymeans ulator stage 26 tingsystem of Fig. `6 by means ofi-the phase-reversing device 2|- which iscapable of producing a pair oi' waves of carrier frequency, but whoserelative phases differ by 'I'he device 2l is further characterized byth? Provision of means for selecting either of `these waves totheexciusionof ,the other, -in response to a control or keying, signal..This keying signal may be derived from the unit 22 which comprises thecircuit means necessary for generating the proper. keying signal inresponse to signals de- The carrier derived from the v be supplied to asuitable phaseand-finally to a suitable' substantially degrees.

i tube V1 is operative, the signal supplied Vto the connected by way ofthe lead 33 to the cathode v impulses whose duration and timingcorrespond to the duration and timing of every second blanking" signal,i. e., to the cross-hatched signals of liigf Reference is now made toFig. 'I in which there is shown a schematic diagram of a, circuitadapted for use inthe controllable phase-reversing means 2| and thekeying signal source 22 of Figr.

-The controllable phase-reversing means 2l il- .l5 lustrated in Fig. 7comprises a pair of vacuum tube amplifiers V1 and V2 having their inputgrids connected in push-pull relation to the balanced secondary windingof the carn'er input transformer 30, and their anodes connected inparallel to the interstage transformer 3l. In the operai tion of thiscircuit the tubes V1 and Vs are diertransformer I8 will be in relativelyreversed phase to the signal supplied in the event that tube V2 isoperative and V1 inoperative. This follows from the fact that the gridsofVi and V2 are connected 3 to opposite ends of the vsplit secondary oftransformer 30, whereas the anodes of Vi and V2 are connected togetherand to the upper or high potential end of the primary winding oftransformer 3|. f

-The keying signal unit 22 is a device for con; trolling `the bias oftubes Vi and 'V2 of unit 2| A in such a manner that a selected one ofthe said tubes is rendered operative while the other is renderedinoperative, or .vice versa. The bias of .4 the tubes V1 and V2 arecontrolled by the tubes Vs and Vs respectively, the cathode'of V1 beingload 34 of the tube V5. Similarly, the cathode of V: is connected by wayofthe lead 35 to the cath- 45 ode load' 36 of tle tube Vs. Neither ofthe-tubes Vo and Va is here provided with a plate circuit load, theirscreens and anodes being connected directly to the positive highpotential `supply terminal B+.

The control grids of tubes V5 and Ve are condenser-coupled to the anodeand cathode loads 31 and 39, respectively, of the` signal invertingldriver tube V4. Preferably, the anode and cathode loads 31 and 38 aresubstantially equal in 55 v f. magnitude so that the signals applied tothe control grids-of Vs and Ve will be not only opposite in phase,butequal in* magnitude as well.,l The operation of tubeV4 may becontrolled by a suitable source oi control pulses represented by therectangle. To carry on the preceding description of a system "forsupplying-a signal of the type described wlthreference to Fig. 5 (b),`the device 29 may comprisea circuit adapted tobe energized by thelbianking signal and capable of 35` the tube V2 inoperative during theperiods ti--ta ta-t4, etc., but operative during the periods tze-ja..

supplying to the control 'grid of V4 a signal similar to the blankingsignal, but having only half the number of pulses per second. In Fig. 7the rectangle 39 has, accordingly, been referred to as an "alternatepulse rejectcr. Circuits capable of longer`intervals-t2-ts, t4 -ts. etc.of Fig. `5 b);

vurotion during the shorter intervenir-a, t` e Under these conditionsthere wuibe' estati nais.

'signal occurring at the rate of sixty per second,

scanuanyto plate-currenteut-off (i. eoring the but `is drivensubstantially to plate-currents t etc., by vthe pulses receivedfromthede across the cathodeload 3B a signal havin wave shape andpolarity (phase) of Atires applied to the grid of'V-i, while across thecathode, f load 34` there will appear asignal havingfflike wave shape,but opposite polarity. These'signals f.

' (i. e. the signals appearing at the, cathodeslgof'Vs and Vo) may thenbe employed as fkeying: signais to differentially render operative` andi'nopjerative tubesVi and V2 in the phase reversingunit 2|, for thepurpose hereinbefore described,`r

It has already been stated that matarme?? H .phase reversals arepreferably suspended during a portion of the vertical synchronizinginterval This is readily accomplished through the :agenc of one of thesignals supplied;A by conventiona generators of standard RMAsynchronizing sig- ,i

This signal is a substantially :rectangular and having a duration ofapproximately nine line periods. vThe signal in questionstarts"v in syn-Accordingly, this 9 -line signal (which can be" derived from unit 21 ofFig. 6) may be` supplied 35 to the keying signal source 22 along withthe modified blanking signal supplied by the alter` nate 4pulsereject-or" 39 of Fig. 7. The signals may be added together-by means of aconventional signal combining circuit, of which many varieties are wellknownin the art. 1

The relation between the blaiking and keying signals, the operation oftubes V1 and V2, and

the phase changes involved in the operation of the specific embodimentillustrated anddescribed f,

with reference to Figs. 5 (b), 6, and '7, may best lbe understood byreferring to the explanatory` drawings of Fig. 8. The various functionshere illustrated are alldrawn against, a common time axis whichfemploysthenotations t1, t2, etc. alreadyusedinFig.5 (12)'. Thefirst Isignalillustrated is the blanking'sig--` nal which is transferred from `thesource 21 to the alternate pulse rejector 39 of Fig. 7. `The output` of`unit 39 is the second signal illustrated in Fig. y8. I'he keying'signal derived from cathode load ze of Vois denoted S- as in Fig. a.The keying ysignal K. S, 34, derived from cathode load 34 of V5,`is'seento `be of similar wave form,- but of opposite polarity.` SignalS.A 3l, which is applied to the cathode of Vi. renders tube Viinoperative during the periods ta--ta` tri-rta, etc. but operativeduring the periods tiftz, ts--ta V36 which is applied to the cathode ofV2, renders tl-f-t's, etc., as represented by thecrosslhatched areas inlineVa of Fig. 8.

`The phase changes undergone byjthecarrier i w'aveare illustrated in thediagram designated r Relative phase.`

During the longer intervals tz-,fta tee-t5, etc., Y the relative phaseof thev carrier maybe regarded as substantially ilxed at some arbitraryvalue 1. -l

between a directly received carried signal. and an `The phase isperiodically reversed, however, and,

accordingly, during the shorter intervals t1-t2,

ts-t4, eto., the relative phase of the carrier is j advanced to a valuec+ 180. Of'course, it might just asv well be retarded toa value -180.These phase shifts are effected during alternate blanking signalimpulses in accordance with the echov cancellation method described withreference to Fig, 5 (b). l o Strictly speaking, the phase of analternating wave varies continuously at .the rate of 360 del grecs percycle. In the present invention, however, we are not concerned with thiskind of phase variation, but rather-with thel phase differenceindirectly received carrier (echo) signal. Consequently, in theforegoing description, and in the claims, the term phase `that concernsus in the description of our invention. Thus, in the function designatedRelative phase in Fig. 8, the regular phase progression from cycle tocycle of the R. F.- carrier is ignored, as are also the less regularphase changes which may occur as a result of carrier frequency drift orvariation, and only the phase reversals produced by the phase reversingmeans 2i of Figs. 6 'and'7 are represented.

While the operation of the apparatus of Figs. 6 and 'I has beendescribed in detail .only with'respect to the echo cancellation methodof Fig.

5 (b), the foregoing example will enable one skilled in the art to putinto practice other echo cancellation methods, such, for example, asthose described with reference to Fig.v 5 (a), (c), (d) and (e). Sincemethods for generating the necessary keying signals in response to theblanking and/on synchronizing signals are well known, it

isnot deemed necessary to refer to the details of this process.- i

Thus far our invention has been described Vwith Particular reference totelevision transmission system in which the video signal is transmittedv Y by amplitude modulating the carrier. I'he invention-is also adaptedfor use, however, in systems wherein the vide'o`signal is transmitted byfre- Quency modulating the carrier. In fact, because of the particularlyobnoxious appearance of the echo images in FM' television transmissionsystems, some means for eliminating or cancelling echoes is especiallytobe desired.

In FM systems the'widths of the echo bars are not xed as they are in anAM system. Instead they vary in width from time to time in accordancewith the changing illumination 'of the picl ture in the screen areaaffected bythe echo. This is because in 'an FM television system theinystantaneous carrier frequency is a function oi' illumination,and,-.consequently, the number oi' 'beats (which produce-the echo bars)between thev delayed synchronizing signal (i. e'. the echo) andV thereceived picture signal varies with .picture illumination. Thus, as theillumination of the picture varies in a given area-e. g. as caused bythe movement of actors or vehicles, or by movement of the televisioncamerathe echo will present an ever-varying-and moving image, whichtbaclluse of its motion is much more objectionable e echoes encounteredin AM-'systems of television.,

'Ihe present invention can greatly reducethe ei'- 70 rect of theseechoes in spite of their movement, because in` general "this movement(of the echo over the screen) takes place slowly enough .so that betweenidentical lines in successive frames there i l is suiilcient similarityas to echo image position 75 afasaoas observer than the Imotionless orfixed to enable an alternating dark-and-light echo in one frame to bereplaced by a substantially correspondingly-placed alternatinglight-and-dark echo in the immediately, following frame. Thus, it willbe seen that our invention is adapted not only to systems wherein theecho produces a substantially fixed pattern upon the picture tubescreen, but also to systems in which the echo pattern. while varyingsubstantially from minute tominute, or second to second, changes onlyslightly, or to a negligible degree, from frame to frame.

Although our invention has been described and illustrated with referenceto certain preferred embodiments, it should be understood that widealterations and modiilcations may be-made with` in the scopefof thisinvention as deiined in the appended claims.

We claim: l 1. In a constant-carrier-frequency television deleteriouseiects of echo signals on the desired `signal, which comprisesperiodically altering the relative carrier phase of selectedcarrierintervals to produce echo images of contrasting characteristics insuccessive frames.

2. In a television system, the method of generating a television signalwhich will ensure substantially echo-free.reception, whichcomprises'generating a carrier wave of constant frequency,

periodically altering the relative phase of said wave in a predeterminedtime sequence, and vmodulating said wave in vaccordance withthefintellif gence to be transmitted.

3. In afIconstant-carrier-frequencyl television transmission system, themethod of reducing the deleterious effects of echo signals on thedesired signal, which comprises phaseat least l 2 times per second inpredetermined sequence prior toy transmission. thereby to cause echoimages ofcomplementary characteristics to appear upon a receiverspicture viewing screen in alternating sequence. where Lis the number ofpicture lines per' frame, and F is the numberl of complete 'framestransmitted per second.

4. In a television transmitting system including a source ofconstant-frequency carrier wave oscillations, apparatus ,for reducingthe deleterious effects of echo signals on the desired signal,comprising controllable means for periodically shifting the relativephaseof saidl oscillations in response vto a control signal, themagnitude of each 'phase shift being substantially electrical degrees,where n is ajsmall integer other l. than l. and-a source of controlsignals connected to said phase'shifting means tocontrol the periodicityof said phase shifts.

5. in a television transmitting system including a source ofconstant-frequency carrier wave os cillations. apparatus forreducing'the deleterious ee'cts of echo signals on the desired signal,com'- prising controllable means for reversing the relative phaseofsaid. oscillations in response to a control signal, means for modulatingthe oscillations i' derived-.from said phase reversingmeans inaccordance with anintelligence signal, a source of .synchronizing andblanking signals, a control signal source connected to and derivingsignals from said second-named source, and a connection bereversing the.carrier asaoss tween said control signal source and said phase reversingmeans for eecting carrier phase reversals in accordance witha'predetermined function of signals derived from said second-namedsource. A

6. In a constant-carrier-frequency television system, avsource ofcarrier wave oscillations, a source of video signals. a source ofsynchronizing signals, modulating means for imparting to said carrierwave the video and synchronizing intelligence from said secondandthird-named sources, means operative during odd frame intervals foreffecting the transmission of a given line at a differ#v ential carrierphase of 0 degrees with respect to the preceding synchronizing pulse,and means operative during even frame intervals for effecting thetransmission of the corresponding line at a dif' ferenti-al carrierphase of @+180 degrees with respect to its preceding synchronizingpulse, where o is an arbitrary and substantially fixed phase angle. l

7. In a constant-carx'ier-frequency television system, a source ofcarrierl wave oscillations, a source of video signals, a source ofsynchronizing signals,` modulating means -for imparting to said carrierwave thevideo and synchronizing intelligence from said secondandthird-named sources,

FRANK J. BINGLEY. l WILLIAM E. BRADLEY.

